1. Field of the Invention
This invention relates to a method for fabricating an yttrium-based high temperature superconducting tape, particularly to a method for fabricating an yttrium-based high temperature superconducting tape preferably usable for a superconducting motor, a superconducting electric generator, an electromagnetic ship, a superconducting linear transportation and a superconducting power applied instrument such as a superconducting magnet for materials science.
2. Description of the Prior Art
Recently, intense research and development have been carried out for obtaining a high temperature superconducting material, and as a result, an yttrium-based high temperature superconducting material such as YBa2Cu3O7 and a bismuth-based high temperature superconducting material such as Bi2Sr2Ca2Cu3O10 have been developed. Since such high temperature superconducting materials have their respective critical temperatures higher than the boiling point of liquid nitrogen, they can exhibit their respective superconducting properties using liquid nitrogen. As a result, such a high temperature superconducting material is expected for various industrial uses.
In the case of applying the above-mentioned high temperature superconducting material to a product to be used in a strong magnetic field, the magnitude of a magnetic field to maintain the superconducting property of the superconducting material is an important factor, and it is desired that the superconducting material can maintain the superconducting property in a strong magnetic field.
When a higher magnetic field is applied to the bismuth-based high temperature superconducting material at a higher temperature, the superconducting material can not maintain the superconducting property. Therefore, for using the bismuth-based high temperature superconducting material in a higher magnetic field, the superconducting material is required to be cooled down to a temperature of 20K or below. On the other hand, an yttrium-based high temperature superconducting material can maintain the superconducting property at a higher temperature under a strong magnetic field, so that it is expected for various applications such as a superconducting magnet and so on.
In the case of employing such a high temperature superconducting material as a product to be used in a strong magnetic field such as a superconducting magnet, it is required to be fabricated as a long-length wire. Since the yttrium-based high temperature superconducting material has small anisotropy, the superconducting property of the material may be degraded largely through the grain boundary-weak binding due to the slight slippage of the orientation when the superconducting material is drawn and two-axially oriented. Therefore, although the yttrium-based high temperature superconducting material can have the superconducting property at a higher temperature in itself, the yttrium-based superconducting material can not be used in a practical use, for example, a superconducting magnet.
Attempt is made to deposit the yttrium-based high temperature superconducting material as follows: First of all, a two-axially oriented buffer layer is formed on a Hastelloy tape by ion-beam technique, and then, a two-axially oriented yttrium-based high temperature superconducting film is formed on the buffer layer, to fabricate a deposited and oriented yttrium-based high temperature superconducting tape. Another attempt is made as follows: First of all, a two-axially oriented buffer layer is made on a nickel tape oriented by a cold-working method of so-called xe2x80x9cRABITSxe2x80x9d, and then, a two-axially oriented yttrium-based high temperature superconducting film is formed on the buffer layer by the same manner as mentioned above, to fabricate a deposited and oriented yttrium-based high temperature conducting tape.
However, since those fabricating methods are very complicated, it is difficult to fabricate even a shorter yttrium-based high temperature superconducting tape. As of now, a longer deposited and superconductivity-maintaining at high degree yttrium-based high temperature conducting film can not be fabricated, so that can not be used in a high magnetic field product such as a superconducting magnet.
It is an object of the present invention to provide a method for fabricating a longer deposited yttrium-based high temperature superconducting tape usable for a high magnetic field product such as a superconducting magnet.
For achieving the above object, this invention relates to a method for fabricating an yttrium-based high temperature superconducting tape, including the steps of preparing a given long member, sticking a non-oriented long silver member on the long member, to make a long base material, and fabricating an yttrium-based high temperature superconducting film by a CVD method with applying a given magnetic field to he film.
In the fabricating method of the present invention, since the non-oriented long silver member serves as an underlayer for the yttrium-based high temperature superconducting film to be made non-orientation, and the yttrium-based high temperature superconducting film is made by a CVD method under a given magnetic field, the yttrium-based high temperature superconducting material is easily oriented in the c-axis along the direction of the applied magnetic field.
Moreover, the magnetic field application enables the crystalline grains to be made smaller and the grain boundary-weak binding to be degraded, so that the resulting yttrium-based high temperature superconducting film can exhibit the superconducting property even at the polycrystal condition, and thus, can be used for a product to be used in a strong magnetic field such as a superconducting magnet. As a result, the natural property of the yttrium-based high temperature superconducting material of the superconductivity being able to be maintained at a higher temperature and a higher magnetic field can be given to a practical product made of the material.
In a preferred embodiment of the present invention, the surface of the long silver member is mirror-polished before the yttrium-based high temperature superconducting film is made. In this case, the orientation of the superconducting film can be more enhanced.
In another preferred embodiment of the present invention, a calcium-including yttrium-based high temperature superconducting film, in which calcium elements are partially substituted for yttrium elements, is formed on the yttrium-based high temperature superconducting film, and then, thermally treated. In this case, the electron concentration in the grain boundaries of the yttrium-based high temperature superconducting film can be developed by the calcium elements of the calcium-including yttrium-based high temperature superconducting film, and thus, the electrical insulating property between the grain boundaries can be weakened.
As a result, the critical current property of the yttrium-based high temperature superconducting tape of the present invention can be largely developed.